Line scanning cameras are well known in the art and have found a number of applications, ranging from document inspection to automated device inspection for quality control purposes. In the latter application, especially with respect to electronic components, there is often a requirement to scan the physical characteristics of a microminiature circuit substrate and its circuit features (e.g., via holes, conductor lines, land areas, etc.). Because of the very dense and miniature configurations being examined, a great number of line scans are required to obtain the necessary feature definition. If the item being scanned contains a number of gray scale levels, they must be accurately portrayed in the final image for the inspection system to be effective. Additionally, in order to prevent the inspection process from taking an inordinately long period of time, the scan process must be carried out at a relatively rapid rate to achieve a reasonable product throughput.
A problem with line scan camera systems results from speed variations of the mechanical means used to provide relative motion between the camera and the workpiece being inspected. When a line scan camera, such as a CCD camera, is employed, the control for the initiation of the scan generally is derived from the mechanical means itself (e.g., a positional encoder attached to an x y table). Such means are often moved under control of a motor or other movement device which is subject to speed variations, jitter or other perturbations. When fine scans are being obtained, these movement variations cause changes in the camera exposure times and thereby result in succeeding lines of the image having different gray scale values. These make automated interpretation difficult. For instance, when very fine pixel (picture element) images are required, if the amplitude of vibration in the movement device approaches the order of a fifth of a pixel size, the derived gray scale level can vary on the order of plus or minus twenty percent.
Others have attempted to cope with this problem in less demanding applications. For instance, in U.S. Pat. No. 4,591,727 to Gaebelein, et al and assigned to the same assignee as is this application, a document scanner is described wherein an exposure time problem exists. Gaebelein teaches measuring the time between exposure pulses through the use of a digital counter, whose count is then employed in a multiplication which compensates for the exposure variation. Since however, Gaebelein's compensation is by multiplication of the digital data word after analog to digital conversion, the accuracy of the compensation is limited to discrete correction levels. For instance, if a one out of 15 analog to digital correction scale is employed, quantization errors of approximately seven percent occur (i.e., one part in fifteen). As fewer levels are used, the quantization error increases.
Another method of attempting to correct the problem is taught by Roth in U.S. Pat. No. 4,396,950. In lieu of attempting to correct the exposure variations as taught by Gaebelein, Roth normalizes his camera output by forcing its exposure time to be constant, but smaller than the shortest scanning interval. This solution obviously limits the efficiency of the system and causes useful exposure time, which would otherwise be available, to be eliminated.
Accordingly, it is an object of this invention to provide an exposure compensation system for a line scanning camera which is both accurate, efficient and simple in construction.
It is another object of this invention to provide a line scanning camera whose output gray scale levels exhibit constant characteristics from scan to scan.